Adjustable dock boards or dock levelers are employed on loading docks to span the gap which exists between the dock base and the bed of the truck. The truck is generally backed into position against the dock for loading and unloading. Dock levelers have a ramp or deck which is mounted usually in a pivotal manner at its rear edge and have at the front edge of the deck an extension lip which is hinged so that it rests on the bed of the truck. Generally, the dock leveler structure is positioned within a shallow pit so that the back hinged edge of the deck will be flush with the dock surface and the lip pendant front of the dock and normally maintained in a position flush with the dock when not in use.
When in use the deck is generally angularly inclined with the deck upward or downward relative to the dock to accommodate trucks of different bed heights. The dock then floats by hinged movement between the lip and the deck to accommodate variations in truck height as the truck is loaded or unloaded.
In operation generally, the truck backs in to position against the front of the dock board. A hold-down device is released so that the deck raises upwardly generally by means of a spring mechanism or the like. This upward action of the dock board causes the extension lip to be raised after it has cleared the rear end of the truck bed. With the deck then in its raised position, an operator generally walks out onto the deck to load it by means of his weight and cause it to move downward into engagement with the bed of the truck. The leveler with its extension lip in position thus bridges the gap between the end of the dock and the carrier bed even though the truck's bed may be above or below dock level.
In normal use, a dock leveler is supported at the outer end by resting on top of a truck. Safety legs are well known devices used to limit the downward travel of a dock leveler in case the truck should move away from under the dock leveler lip and allow it to fall. The safety legs are usually designed to provide at least two stop positions between dock level and the minimum downward position. When the safety leg is used to support the dock at the level position, it is often known also as a cross-traffic support.
The art is replete with a number of different configurations. Reference is made to the following U.S. Patents which describe such configurations some having safety legs.
U.S. Pat. Nos. 3,137,017, 3,299,456, 3,368,229, 3,530,488, 3,728,753, 3,835,497, 3,858,264, 3,877,102, 3,902,213, 3,921,241, 3,995,342, 4,104,059, 4,126,909, 4,279,050, 4,328,602, 4,455,702, 4,619,008.
A common deficiency with these prior art devices is their inability to reliably hold down a mechanically operated dock leveler and yet permit acceptable "float". Generally, the dock assembly is spring loaded so that when a hold-down device is released the deck will rise and the lip will extend. Given this bias in the system to mechanically raise the deck, a need exists to provide a technique which will hold down the deck in contact with the truck bed as the truck "floats" as a function of the loading or unloading operation.
Another deficiency of the prior art relates to the technique of compensating for motion of the truck as it rises on its suspension while being unloaded. Modern trucks having an air suspension may rise up to ten inches as a load is removed. Most hold-down devices compensate for this motion by extending against the spring. This in turn creates high loads on the hold-down device as the spring is deflected.
Yet another deficiency in the prior art is the use of cables or plastic straps which are not rugged or reliable. The area of use of a dock leveler, loading dock, is notorious for its dirt, contamination, hazards. Reliability in rugged operation is a requirement of equipment used in that environment. Other hold-down devices employ one-way ratchets and bar designs which also employ a spring to compensate for the rising of a truck bed as it is unloaded. This again causes very high loads on the hold-down device.
In order for the dock leveler to operate below the level position, the safety legs must be pivoted away from the position where they would engage the stops on the lower frame. This is usually accomplished by pulling a chain or other device. However, when the dock leveler is operating below dock, the safety legs often re-engage the stops when not desired. The reason for this is that each time a fork truck travels back and forth into and out of the truck, the suspension of the truck can deflect several inches due to the change in load. Since the dock leveler is supported by the truck, it must follow that motion. If the dock leveler is operating well above the level of the dock, there is little likelihood that the deflection will be so great that the leveler will travel low enough to engage the safety legs against their stops. However, if the dock leveler is operating in a position which is very close to the safety leg stop, the up and down motion of the truck can cause the safety legs to slam down on the stops. Although the safety legs are designed to withstand heavy loads, the repeated pounding is potentially damaging to both the fork truck and the dock leveler, and is objectionable from the view point of noise and operator comfort. A similar problem can occur when the dock leveler is operating at a position which is very close to where the safety legs can re-engage. The dock leveler may rise above that point, allow the legs to re-engage the stops, and the next time the fork truck enters the truck and the suspension deflects, the dock leveler will slam down against the safety legs rather than be supported by the truck.